(1-x^2)(1-y^2)-4xy
=1-x^2-y^2+x^2y^2 -- -- -- -- ^2代表平方
=(-x^2-y^2-2xy)+(x^2y^2-2xy+1)
= -(x+y)^2+(xy-1)^2
= (xy-1+x+y) (xy-1-x-y)